1. Field of the Invention
The present invention relates to a variable-magnification optical system for use in a lens unit or the like, and to an image-taking apparatus incorporating such a variable-magnification optical system.
2. Description of Related Art
With the recent spread of personal computers (PCs), there has been increasing popularity in digital cameras (image-taking apparatuses), which permit easy capturing of images. In such digital cameras, as in cameras using silver-halide film (silver-halide film cameras), compactness (slimness) and high-performance (for example, high zooming and aberration-correcting capabilities) are eagerly sought for.
To meet these requirements, there have been developed image-taking apparatuses (video cameras and the like) that incorporate a variable-magnification optical system (zoom lens system) that includes a rectangular prism in the lens group (first lens group) closest to the object side. Examples of such image-taking apparatuses are disclosed in, to name a few, Japanese Patent Application Laid-open No. H8-248318, laid-open on Sep. 27, 1996 (hereinafter Patent Publication 1) and Japanese Patent Application Laid-open No. H9-146000, laid-open on Jun. 6, 1997 (hereinafter Patent Publication 2).
In these image-taking apparatuses, the rectangular prism bends the optical axis, and thereby serves to reduce the length of the first lens group, and thus the total length of the zoom lens system. This permits a zoom lens system with a so reduced total length to be arranged within the limited space inside the housing of an image-taking apparatus, and accordingly helps make the housing, and thus the image-taking apparatus itself, compact and slim.
The image-taking apparatuses of Patent Publications 1 and 2, however, have the following disadvantages. These image-taking apparatuses incorporate a variable-magnification optical system composed of a plurality of lens groups arranged in a positive-negative-positive-positive optical power arrangement. Thus, to achieve magnification variation (zooming), for example, the second lens group needs to be moved through a comparatively long distance.
Here, the distance through which the second lens group needs to be moved may make the variable-magnification optical system (zoom lens system) unduly long (make its total length unduly large). As a result, the image-taking apparatuses of Patent Publications 1 and 2 cannot be said to be made satisfactorily compact or otherwise favorably constructed.